Page 106 - Programmable Logic Controllers, Fifth Edition - Mobile version

L1 Inputs L2 L1 Outputs L2 A B D
PB1 C
SOL1
I:4/5 O:2/3

PL1 Figure 5-30 Parallel input branches.
LS1
I:4/6 O:3/6 R
A C

Figure 5-28 I/O connection diagram. D
B
Allen-Bradley Logix 5000 controllers offer a more E
flexible method of addressing memory space. Instead of
a fixed device with a fixed address space, tags are used
for assigning and referencing memory spaces. Tags are a Figure 5-31 Parallel output branches.
pure text based addressing scheme and a departure from
the more conventional ways of programming PLCs.
The assignment of an I/O address can be included in illustrated in Figure 5-31. When there is a true logic rung
the I/O connection diagram, as shown in Figure 5-28. In- path, all parallel outputs become true. In the example
puts and outputs are typically represented by squares and shown, either A or B provides a true logical path to all
diamonds, respectively. three output instructions: C, D, and E.
Additional input logic instructions (conditions) can be
5.6 Branch Instructions programmed in the output branches to enhance conditional
control of the outputs. When there is a true logic path, in-
Branch instructions are used to create parallel paths of cluding extra input conditions on an output branch, that
input condition instructions. This allows more than one branch becomes true. In the example shown in Figure 5-32,
combination of input conditions (OR logic) to establish either A and D or B and D provide a true logic path to E.
logic continuity in a rung. Figure 5-29 illustrates a typical Input and output branches can be nested to avoid re-
branch instruction. The rung will be true if either instruc- dundant instructions and to speed up processor scan time.
tion A or B is true. Figure 5-33 illustrates nested input and output branches.
Input branching by formation of parallel branches can A nested branch starts or ends within another branch.
be used in your application program to allow more than In some PLC models, the programming of a branch
one combination of input conditions. If at least one of these circuit within a branch circuit or a nested branch can-
parallel branches forms a true logic path, the rung logic is not be done directly. It is possible, however, to program
true and the output will be energized. If none of the parallel
branches complete a logical path, logic rung continuity is
not established and the output will be de-energized. In the A C
example shown in Figure 5-30, either A and B, or C pro-
vides logical continuity and energizes output D. B D E
On most PLC models, branches can be established at
both input and output portions of a rung. With output
branching, you can program parallel outputs on a rung Figure 5-32 Parallel output branching with conditions.
to allow a true logic path to control multiple outputs, as

A C

B

Figure 5-29 Typical branch instruction. Figure 5-33 Nested input and output branches.

Basics of PLC Programming Chapter 5 87

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